The invention relates to flame retardant cable for telecommunications. More particularly, the invention is directed to an insulated conductor having at least two insulation layers, a composite dielectric constant less than or equal to about 2.6, and a composite dissipation factor less than or equal to about 0.005. Telecommunications cable prepared with the insulated conductors is particularly well-suited for use in plenum spaces of air circulation systems.
In the construction of buildings, it is extremely important to use materials which resist the spread of flame and the generation and spread of smoke in case of fire. Accordingly, it is important to select and install telecommunications cable meeting these material requirements.
Conventional designs of telecommunications cable for plenum chambers of air circulation systems generally use polyvinyl chloride or fluoropolymers as jacketing materials. Also, most plenum cable insulation development has focused on improving flame retardance and reducing smoke generation using expensive fluorinated ethylene polymer (“FEP”) or polyolefins having high levels of flame-retardant or smoke-suppressing additives. Unfortunately, the additives degrade the dielectric properties of the polyolefins. Also, when the additives are halogenated, they generate corrosive gases during combustion.
TIA/EIA 568A standards set electrical requirements for category 5 and 5e cables, such as impedance, attenuation, return loss and crosstalk requirements for any given conductor pair in a frequency range of 0.772 to 100 MHz. 100 meters of cable are used for this test. Impedance should not be less than 85 at 0.772 MHz and no more than 115 at 100 MHz. Return loss should be no less than 17 to 25 dB, depending on the testing frequency of 0.772 to 100 MHz. Attenuation should not be higher than 5.8 to 22 dB, depending on testing frequency of 0.772 MHz to 100 MHz.
Impedance and attenuation are important electrical properties. Impedance is the resistance to signal transmission along the length of the cable. The impedance of cable is controlled by conductor diameter and its properties, type of insulation used and its thickness, and tightness with which individual pairs are twisted. Thicker cables give higher impedance. But if insulation is too thick, the cable impedance can exceed the maximum desired value.
Attenuation is the reduction in signal strength over the distance the signal is transmitted. Conductor and insulation are the major contributors to the cable attenuation. The larger conductor or lower resistance gives lower attenuation. The greater insulation thickness also gives lower attenuation.
With a given cable design, the desired dielectric properties of insulation are to have dielectric constant (DC) no more than 2.6 and dissipation factor (DF) no more than 0.005 at 1 MHz, per ASTM D 1531 test method, to meet Category 5/5e electrical requirements on cables.
The NFPA 262 test is a fire test method to determine flame propagation distance and optical smoke density for electrical and optical-fiber cables intended to be installed in ducts, plenums, and other spaces used to transport environmental air without the cable enclosed in raceways. Cables are placed in a cable tray with 25-foot long cables covering the entire tray width of 11.25 inches. An ignition burner flame (88 kW) is applied to the one end of the cable tray with an air velocity of 240 ft/min. The test duration is 20 minutes. The cable will be considered as a pass if the flame spread is no more than 5 ft., the peak smoke is no more than 0.5, and the average smoke is no more than 0.15. A cable is flame retardant if it meets the latter criteria.
U.S. Pat. No. 5,462,803 describes a cable with conductor with fluoropolymer surrounding a conductor and an outer layer of polyvinyl chloride.
U.S. Pat. No. 5,563,377 describes using an inner layer of inexpensive “flame retardant polyolefin” and an outer layer containing FEP. Chlorinated, brominated or metal additives are used to impart flame retardancy to the polyolefin.
Canadian Application 2,238,596 describes a foamed flame retardant inner layer made from a polyolefin and flame-retardant additives.
U.S. Pat. No. 5,670,748 to Gingue, et al. describes a conductor surrounded by a foamed layer of polyolefin or polyurethane and an outer layer of halogenated material.